Polyimides constitute a class of valuable polymers being characterized by thermal stability, inert character, usual insolubility in even strong solvents, and high Tg, among others. Their precursors are usually polyamic acids, which may take the final imidized form either by thermal or by chemical treatment.
Polyimides have always found a large number of applications requiring the aforementioned characteristics in numerous industries, and recently their applications have started increasing dramatically in electronic devices, especially as dielectrics. With continuously escalating sophistication in such devices, the demands on the properties and the property control are becoming rather vexatious.
Especially for the electronics industry, improvements of polyimides are needed in forming tough, pin-hole free coatings, having lower dielectric constant, lower coefficient of thermal expansion, lower moisture absorption, and decreased brittleness, among others. Although it is not usually possible to maximize all properties, since many of them may be antagonistic, optimization as a total is highly desirable and it may be achieved if adequate control on the properties becomes available through molecular architecture or other means.
Different aspects regarding polyimides and copolyimides may be found in a number of publications, such as for example:
Sroog, C. E., J. Polymer Sci.: Part C, No. 16 1191(1967).
Sroog, C. E., J. Polymer Sci.: Macromolecular Reviews, Vol. 11, 161 (1976).
Jensen, R. J. and Lai, J. H., "Polyimides: Chemistry, Processing, and Application for Microelectronics" in "Polymers for Electronic Applications", Lai, J. H., Ed., Ch. 2, p. 33 CRC Press, Boca Raton, Fla. (1989).
Soluble polyimides are described in the following references.
E. S. Moyer, D. K. Mohanty, C. A. Arnold, J. E. McGrath; "Synthesis and Characterization of Soluble Polyimide Homo- and Copolymers"; Polymeric Materials; Science & Engineering Proceedings of ACS Division of Polymeric Materials; V60; p. 202-205; Spring 1989.
M. E. Rodgers, C. A. Arnold, J. E. McGrath; "Soluble, Processable Polyimide Homopolymers and Copolymers"; Polymer Preprints, ACS Division of Polymer Chemistry; V30-1; p. 296; 1989.
Y. Oishi, M. Xie, M. Kakimuto, Yoshio; "Synthesis and Characterization of Soluble Arumatic Polyamides and Polyimides from 1,1-(Bis(4-Aminophenyl)-2,2-diphenylethylene"; Polymeric Materials; Science & Engineering Proceedings of ACS Division of Polymeric Materials; V60; p. 757-761; Spring 1989.
F. W. Harris, Y. Sakaguchi; "Soluble Aromatic Polyimides Derived from New Phenylated Diamines"; Polymeric Materials; Science & Engineering Proceedings of ACS Division of Polymeric Materials; V60; p. 187-192; Spring 1989.
One of the problems with soluble polyimides is that in order to achieve solubility, other properties conventionally are sacrificed, such as for example solvent resistance, thermooxidative stability, and the like.
It has been long known that the properties of a polymer may be best controlled and diversified by using segmented or block copolymers (the words "block" and "segment" regarding copolymers are used in this discussion as synonyms), wherein each of the segments or blocks provides a special and desirable character or property. A classic example is that of the styrene/butadiene block copolymers, wherein the styrene blocks provide stiffness and the butadiene blocks provide elasticity, stiffness and elasticity being two major components of toughness. The desired mechanical properties realized by block polymerization of the above segments cannot be received by random polymerization, despite the fact that the empirical formula, molecular weight, and other parameters may be kept constant in both cases.
Thus, a large number of attempts have been made to duplicate this concept in the case of polyimides, in order to control their properties to better fit the requirements of a given specific application. However, all these attempts have been either partially or totally unsuccessful, due to certain factors, which are more or less unique to conventional polyimide structure and chemistry, especially when combined with the two major contributing facts, among others, as explained hereinafter.
First, polyimides are valuable because they are normally insoluble, as already mentioned. Therefore, they also possess high solvent resistance. However, this beneficial property itself becomes a heavy burden regarding the way to apply a highly insoluble polyimide in the form of a coating, for example. Thus, the most common technique of applying polyimides as coatings is to use a solution of the respective polyamic acid, which is considerably more soluble, and then after the application, convert the polyamic acid to the corresponding imide by either heat or chemical means. An alternate way, also useful in the preparation of segmented polyimides, is to employ soluble oligomers, which have functional terminal groups, such as for example isocyanates, epoxides, ethylenically unsaturated groups, and the like, and then extend them or crosslink them. These functional groups, however, are source of decreased thermooxidative stability, and they may cause in general deterioration of polymer properties.
Second, a special characteristic of poly(amic acids), which are for all practical purposes the reaction products of carboxylic acid dianhydrides with diamines, is that they are perpetually in a status of dynamic equilibrium, in a way that their components (diamines and dianhydrides) continuously interchange positions, depending on the factors which drive said equilibrium, in contrast with polyimides, which typically do not undergo such changes. Poly(amic acid) equilibration is further detailed by C. C. Walker, "J. Polym. Sci.; PART A: Polym. Chem. Ed.", 26, 1649 (1988). Reequilibration of binary poly(amic acid) mixtures is discussed by M. Ree, D. Y. Yoon, W. Volksen; "Miscibility Behavior and Reequilibration of Binary Poly(Amic Acid) Mixtures"; Polymeric Materials; Science & Engineering Proceedings of ACS Division of Polymeric Materials; V60; p.179-182; Spring 1989. On the other hand, equilibration in the case of aromatic polyimides requires stringent conditions, such as for example described by Takekoshi, T., "Synthesis of Polyetherimides by Transimidization Reaction", preprints of symposium on Recent Advances in Polyimides and Other High Performance Polymers, Div. of Polymer Chemistry, Am. Chem. Soc., San Diego, Calif., Jan. 1990.
One way to retard the dynamic equilibration in the case of polyamic acids is to cap the end-groups, for example with reactive groups, as aforementioned in the case of oligoimides. However, the same disadvantages are here also present. An alternate way is to start with an oligomeric amic acid ester, which unlike the amic acid does not substantially undergo interactive interchange.
A functionally terminated oligomeric acid ester can be reacted with diamine moieties in the presence of condensing agents or through the carbonyl chloride adduct of the esterified oligoamide. Some of these methods give low molecular weight polymers, and therefore, downgraded properties, while other methods are very cumbersome, and their by-products have the potential of being corrosive, a fact which is entirely unacceptable in the Electronics Industry. A summary af such methods is given by W. Volkso in "Symposium on Recent Advances in Polyimides and other High Performance Polymers", by the Division of Polymer Chemistry of the American Chemical Society, San Diego, Calif., Jan. 2214 25, 1990, page C1.
These important facts, among others, make the control of the structure of segmented polyimides particularly difficult.
Japanese Patent Publication 63-314241, published on Dec. 22, 1988, describes a method of production of polyacid amide copolymers in which an aromatic diamine is allowed to react with an aromatic tetracarboxylic acid dianhydride to form an acid amide prepolymer. A second aromatic diamine is then added to the solution of the prepolymer, a well as an additional aromatic tetracarboxylic acid anhydride, and the mixture is allowed to react.
Japanese Patent Publication 63-314242, Kamai et al., published Dec. 22, 1988, describes a method of producing a polyimide copolymer film by imidizing a polyamic acid copolymer film, by either heating or chemically treating the polyamic acid copolymer film.
Japanese Patent Publication 63-254131, Mitsubishi Denki KK, issued 10/4/87, describes production of an aromatic polyimide having good molding properties by reacting p-phenylenediamine with pyromellitic acid derivative, and then with aromatic diamine and aromatic tetracarboxylic acid derivative.
Japanese Patent Publication 59-179523, Agency of Ind Sci Tech, issued 31/03/83, describes a thermosetting heat-resistant resin made by heating an imide compound having unsaturated end-groups with an aromatic amine, and optionally with an aromatic tetracarboxylic acid (or reactive derivative).
Japanese Patent Publication 59-232149, describes a composition, which is obtained by incorporating a salt of polycarboxylic acid and polyamine in a polyimide polymer or a polyamide polymer
U.S. Pat. No. 4,410,664, Lee, issued 10/18/83, describes the preparation of a polyimide-epoxy thermoset resin carried out by reacting a polyepoxide with a polyimide dianhydride together with a polyimide dianhydride and/or polyimide diamine, wherein at least one of the polyimide components is insoluble in the solvent in the absence of the other polyimide components.
U.S. Pat. No. 4,197,397, D'Alelio, issued 4/8/80, describes aromatic polyimides with anhydride end groups, chain extended (molecular weight increased) by reacting them with aromatic diamines or diamine terminated imide oligomers. The reaction can either be at a temperature above the melting point of the reactants or in solvents of the reactants. These polyimides can be shaped and formed prior to chain extending.
U S. Pat. No. 4,058,505, D'Alelio, issued 11/15/77 describes an aromatic polyimide with amine end groups, chain-extended (molecular weight increased) by reacting them with aromatic di- or tri- anhydrides. The reaction can either be at a temperature above the melting point of the reactants or in solvents for the reactants. The polyimides can be shaped and formed prior to chain-extending.
In contrast to other unsuccessful attempts, the present invention combines appropriate advantages from both polyimide and polyamic acid chemistries in order to provide highly improved soluble precursors of eventually segmented, preferably insoluble, polyimide copolymers.